1. Field of the Invention
The present invention relates to a communication device which transmits a large volume of data in a proximate distance through a weak UWB communication method using a high frequency broadband, and more particularly to a communication device which employs a weak UWB communication using an electric field coupling and secures a communicationable range in the transverse direction.
2. Description of the Related Art
A noncontact communication method has been widely used as a medium for authentication information or other value information such as electronic money. Also, in recent years, examples of new applications of a noncontact communication system include a large volume data transmission such as downloading or streaming of video, music, or the like. The large volume data transmission is completed by a single user as well, further is preferably completed with the same sense of access time as the authentication and billing process in the related art, and thus it is necessary to increase the communication rate.
A general RFID specification uses the 13.56 MHz band and is a proximity type (from 0 to 10 cm) noncontact bidirectional communication which employs electromagnetic induction as a main principle, but the communication rate is only 106 kbps to 424 kbps. In contrast, as a proximity wireless transmission technique applicable to high speed communication, there is TransferJet (for example, see Japanese Patent No. 4345849 and www.transferjet.org/en/index.html (searched on Mar. 2, 2010). This proximity wireless transmission technique (TransferJet) employs a method of transmitting signals using an electric field coupling action, wherein a high frequency coupler of the communication device includes a communication circuit unit which processes high frequency signals, a coupling electrode which is disposed spaced apart from a ground with a certain height, and a resonance unit which effectively supplies high frequency signals to the coupling electrode.
A proximity wireless transmission using a weak UWB employs a fundamental principle which mainly uses a longitudinal wave component ER which does not include a radiation electric field in an electric field generated by a coupling electrode (described later), has a communication distance of 2 to 3 cm, does not have a polarized wave, has almost the same area in the longitudinal direction and the transverse direction, and has a communicationable range of nearly hemispherical shape. For this reason, between communication devices which perform data transmission, it is necessary to appropriately face coupling electrodes to each other and to cause a sufficient electric field coupling to act on each other.
If the proximity wireless transmission function is manufactured in a small size, it is suitable for built-in use, and, for example, it can be mounted in a variety of information devices such as a personal computer or a portable telephone. However, if the coupling electrode of the high frequency coupler is reduced in size, particularly, there is a problem in that the communicationable range in the transverse direction decreases. For example, if a target point mark indicating a part where the high frequency coupler is implanted is added onto a surface of a case of an information device, it is convenient for a user to adjust the position targeting the target point. However, in a case where the communicationable range in the transverse direction is narrow, when the devices come close to each other, there are some cases where the target point is hidden in shadow, shifted from the center position in the transverse direction and touched.
In order to heighten the practical use of the proximity wireless transmission function, it is necessary to expand the communicationable range in the transverse direction. However, if the size of the coupling electrode of the high frequency coupler is simply increased, a standing wave is generated on the surface of the coupling electrode. Further, in a part where the amplitude of the standing wave is reversed, charges having different polarity are distributed, and the neighboring charges having the different polarities mutually cancel the electric fields of each other, and thereby a place where the field intensity is strong and a place where the field intensity is weak are generated. The place where the field intensity is weak becomes a insensitive point (null point) in which it is difficult to obtain a good electric field coupling action even if a coupling electrode of a communication partner comes close thereto.
On the other hand, according to an electric wave communication method using radiation electric field, the communicationable range can be greatly expanded. However, there is no sharp operability for specifying a communication partner in an act of a device coming close to a target point. In addition, it is necessary to take into consideration the preventing hacking or of securing confidentiality on a transmission path.